1. Field of the Invention:
The present invention relates to a method for forming an oxynitride layer, in particular, a method for forming an oxynitride layer to avoid boron penetration.
2. Description of the Prior Art:
In metal oxide semiconductor field effect transistors (MOSFETs), gate oxide plays a very important role, as its thickness must be continuously decreased to lower threshold voltage so that speed and current of the device are increased. Under these circumstances, boron penetration problems easily occur as the size of devices continuously decreases, especially PMOSFET with P+ polysilicon gates, wherein CMOS thereof must have low and symmetrical threshold voltage to achieve operation in low voltage. However, boron ions from the P+ polysilicon gate easily penetrate to the silicon substrate at the bottom. This not only shifts the threshold voltage to the positive direction, but also lowers the sub-threshold swing, which seriously affects the characteristics of time-dependent dielectric breakdown. Consequently, boron penetration must be overcome to obtain semiconductor devices that meet the operational requirements.
A few research papers have disclosed methods for improving oxynitride layers, such as forming an NH3-rich layer so that nitrogen accumulates at the surface of the oxide layer and the interface of oxide layer and silicon substrate, thereby inhibiting dopant diffusion. For hydrogen-related species, however, electron trapping problems can occur.
Another method discloses using N2O instead of NH3, but high thermal budget must be obtained to provide sufficient nitrogen concentration to achieve inhibition of boron penetration. If the thickness of the oxide layer is less than 35 xc3x85, using N2O cannot effectively solve the boron penetration problem. In another method, using NO, although there is no thermal budget problem as above, the maximum value of the nitrogen concentration located at the interface brings fixed-charge buildup. This, more or less, results in boron penetration to the oxide layer, which consequently affects the performance of the device. The optimum gate oxide layer must have a nitrogen diffusion barrier at the interface, and avoid boron penetration at the same time. A good oxide interfacial quality is then sufficient to maintain carrier mobility and transconductance.
In order to achieve the nitrogen profile, i.e. high nitrogen concentration at the interface, methods disclosed are nitridation of thermal oxide layer using high density of N2  plasma, and N2  ion implanting before oxidizing. However, nitridation with high density of N2  plasma is not compatible with the current process, and the latter method is more complicated, thus difficult to integrate with the current process. Moreover, defects associated with ion implantation can affect the quality of the oxide layer.
In order to overcome the above problems, an object of the invention is to provide a method for forming an oxynitride layer, using dry oxidation to form an oxynitride layer in a low-pressure state. Not only are the problems associated with boron penetration inhibited, promoting device quality, the method provided is also compatible with the current process.
In order to achieve the above objects, there is provided a method for forming an oxynitride layer, comprising: (a) providing a substrate and removing the native oxide layer; (b) forming a nitride layer on the substrate; (c) oxidizing the nitride layer to form an oxynitride layer; and (d) subjecting the oxynitride layer to in-situ annealing.
Native oxide layer removal in step (a) is accomplished by buffered oxide etching (BOE) solution, such as HF or NH4F. In step (b), the nitride layer is formed by thermal nitridation, where the parameters are as follows: pressure less than 10 torr, in 800xcx9c1000xc2x0 C. of NH3  gas. The nitride layer formed is 10xcx9c15 xc3x85 thick, preferably 13 xc3x85. Oxidation of nitride layer in step (c) is carried out in atmospheric O2, where the temperature is 900xcx9c950xc2x0 C. In-situ annealing in step (d) is performed in N2  gas, where the temperature is 900xc2x0 C. The oxynitride formed is 25xcx9c30 xc3x85 thick, preferably 28 xc3x85.
According to the invention, the peak value (5.11E21 atoms/cm3) of nitrogen concentration in the oxynitride layer formed is located at the interface of the oxynitride layer, i.e. the interface with the polysilicon layer formed thereafter. The peak value has shifted from the interface of oxide layer and the substrate to the interface of polysilicon/oxynitride. Consequently, it inhibits boron penetrates from the P+ polysilicon electrode into the gate oxide layer. Nitrogen concentration at the interface of the oxynitride layer also enhances the overall reliability of the device.
In addition, based on the results observed by Secondary Ion Mass Spectroscopy (SIMS), it is considered that the profile of nitrogen distributed in theoxynitride layer not only inhibits boron penetration, but also achieves accurate control of the thickness of the oxynitride layer.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.